Electrical machines are utilized in a wide range of environments and for a large number of purposes. Of particular interest with regard to aspects of the present invention is utilization of electrical machines within gas turbine engines. These electrical machines are utilized for provision of electrical generating capacity as well as in order to act as electrical motors for various functions. There is a desire to locate such electrical machines as closely as possibly and preferably embedded within the gas turbine engine. By such an approach, complex mechanical transmission mechanisms are removed giving significant operational and efficiency benefits.
As indicated above, such electrical machines can act as generators as well as starter motors for the gas turbine engine. The electrical machine in normal operation as indicated would generally act as an electrical generator but during such starting functions it will be understood that the electrical machine will temporarily act as a motor. Generation systems typically utilize power electronic converters to enable very high power density electrical machine technologies to be used. Such technologies include switched reluctance electrical machines. High densities enable further size and weight benefits to be achieved in comparison with traditional synchronous electrical machines. Additionally, the inherent direct electrical current interface allows parallel connection and sharing of electrical power generated by different spools of the gas turbine engine.
Whilst there are significant benefits with regard to embedding electrical machines within a gas turbine engine, it will also be understood that by such location it is necessary for the machines to have a high level of reliability and availability. The robustness of switched reluctance electrical machines has attractions but the necessity of a large electrical capacitance for the direct electrical bus interface can create significant size and weight problems. The traditional approach with regard to land based electrical machines is to use an electrolytic capacitor but such capacitors are not conveniently available for aerospace situations due to their limitations in terms of operating temperature, capacitance stability, a limited life and orientation. In such circumstances, less capacitively dense technologies, such as film-foil, are required but in view of the lower capacitive density, it will be appreciated that there are large weight and size penalties incurred.